The membranes separating intracellular parasites from the cytoplasm of their host cell are the site of principal interactions between the two organisms. We propose to continue studies on the origin, nature and function of these membranes in two different types of erythrocytic parasites: malaria parasites, separated from the host cell by two closely apposed membranes; and a species of Babesia, separated from its host cell by only one membrane. Methods of electron microscopy and cytochemistry at the ultrastructure level will be applied to the parasites within erythrocytes and to parasites separated from their host cells. At the same time it should be possible to acquire information concerning the various membrane systems of these parasites. With the availability of the human malaria parasite Plasmodium falciparum in culture, the work has now been extended to this species.